Vessels or bulbs to enclose discharges or filaments, particularly high-pressure discharge lamps and halogen incandescent lamps, are subject to high thermal loading. To make such vessels, light-transmissive ceramics can be used, as well as quartz glass. Pure quartz glass which has a purity of up to about 99.99 mol-% silicic acid is transparent not only for visible light, but also for ultraviolet (UV) radiation. It is necessary to substantially attenuate UV radiation, which is, as radiated, a health hazard. One possibility is to dope the quartz glass which is used as the envelope or bulb for a discharge or for halogen incandescent lamps with suitable dopings which substantially reduce the emitted UV radiation to a safe level. Selection of doping materials, as well as concentration thereof, requires care since the physical characteristics of the quartz glass, for example viscosity, transparency, coloring of the glass, and tendency to crystallization, should not disadvantageously affect the characteristics of the lamp by the doping. Doping materials which are suitable are, primarily, cerium, added as an oxide, a silicate or an aluminate to the quartz powder which is prepared prior to melting the powder to form the quartz glass. A small further addition of titanium, added in the form of titanium oxide, additionally attenuates the particularly dangerous short-wave portion of the UV radiation.
The referenced U.S. Pat. No. 5,196,759, Parham et al., describes a quartz glass which is doped with up to 0.5% cerium oxide and additionally with titanium oxide. The cerium oxide corresponds to a pure cerium proportion of about 0.41%, by weight.
European 0 478 059 A1, van Hal, describes a quartz glass having a UV radiation absorbing doping formed of 0.1 mol-% cerium disilicate and 0.01 mol-% titanium oxide. This corresponds to a pure cerium portion within the quartz glass of about 0.47%, by weight.
Quartz glass can be doped with a higher proportion of cerium, and quartz glass with such higher cerium concentration is described in the referenced application U.S. Ser. No. 08/120,729, U.S. Pat. No. 5,464,462, issued Nov. 7, 1995, Langer et al., assigned to the assignee of the present application. Higher doping with cerium ensures that the dangerous UV radiation is sufficiently absorbed even if the bulbs or vessels are very thin. Cerium aluminate and titanium oxide are described in that application.
The absorption edge of the quartz glass is set to a wavelength of about 350 nm by such cerium-titanium doping. This reduces the transparency of the quartz glass for the undesired, potentially dangerous UV radiation to a tolerable level. Any remanent UV transparency of the quartz glass at wavelengths in the region of about 245 nm can be removed by glowing or annealing the quartz glass for several hours in an O.sub.2 atmosphere.
The cerium in the glass emits a blue fluorescent radiation, stimulated by the UV radiation. This blue radiation can be utilized to improve the color rendition of electrical lamps within the blue spectral range, as described in the above-referenced publications and the application. In some uses, however, such additional blue component is not desired. For example, when using high-pressure discharge lamps in vehicular headlights, such increase of blue light component is undesired. What fluoresces is not the filament but the envelope or bulb, that is, the cerium in the bulb. When such a bulb is inserted in a reflector, or a similar optical system with specifically directed light emission, the blue fluorescence leads to an increase in stray light, which spreads the otherwise sharp light/dark boundary of the desired emitted light beam. For applications where only light from the emitted light source is desired, the fluorescent radiation of the cerium is undesired.
European 0 032 763 B1, to which U.S. Pat. No. 4,361,779 corresponds, van der Steen et al., describes a quartz-glass having a doping which suppresses UV radiation, so that the glass has 0.1 to 3% alkali metal oxide, 0.2 to 5% of a rare-earth metal oxide, and 0 to 0.5% of an alkaline earth metal oxide. Praseodymium oxide (PRO.sub.2) or europium oxide (Eu.sub.2 O.sub.3) are proposed; the alkali metal oxide is listed as potassium oxide (K.sub.2 O) in the examples. The rare-earth metal oxide functions as an absorber for UV radiation. The alkali metal oxide enhances the solubility of the rare-earth metal oxide in the quartz glass. The so doped quartz glass has an absorption edge at a wavelength of about 250 nm, that is, radiation with a wave length below 250 nm is absorbed in the quartz glass; the quartz glass is transparent for radiation having a wavelength higher than 250 nm. The UV radiation in the wavelength range of between 350 nm and 250 nm is transmitted with hardly any attenuation. Consequently, this quartz glass is entirely unsuitable as a bulb or a discharge vessel enclosure for high-pressure discharge lamps, nor for an outer envelope or shield therefor. Besides these dopings, UV radiation with a wavelength of above 250 nm must also be suppressed.